Drill stem safety valves (“DSSV”) typically have two primary purposes: a) they are a safety device that can be closed to prevent mud and/or well fluid from flowing back up the interior of the drill pipe in the event of an unbalanced pressure in the mud column; and b) they can be used as a flow control device to turn on and off the flow of mud while making and breaking connections during drilling operations for top drives. When used for blow out prevention, these valves are only used during testing or in emergencies. However, in mud control, they can be operated several hundred times in the drilling of a single well.
To operate a DSSV, the stem is turned ninety degrees from open to closed position and back again, by applying torque to the DSSV stem. This torque can be applied manually, or by remote actuator. For mud saving operations, remote actuation is the preferred method of applying torque to the DSSV. Remote actuators generally deliver the torque to the stem of a valve through a hexagonal or square shaft that interfaces with the matching internal profile of the stem.
When the valve is used for blowout prevention, the valve can be subjected to high internal pressure which causes a significant amount of compressive load on the valve ball as it moves from open to close. This high load necessitates the application of high torque to the valve stem in order to ensure that the ball completely closes and fully stops the unwanted flow reversal. Some valves require upwards of 2000 ft-lbs to operate. A remote actuator is the most efficient method for delivering the high torque required.
Actuators generally supply a fixed amount of torque, i.e. the maximum output of the actuator. The high torque delivered to valve stem can damage the internal stops for the valve stems. This damage generally leads to over travel of the ball in the open, close, or both positions. This over travel can be detrimental to the life of the valve and the safety that it is supposed to provide. For example, when the ball over travels in the open position, the flow of mud is directed off the longitudinal axis of the valve leading to accelerated wash of the valve's internal components. When the ball over travels in the close position, the valve ball may rotate to the extent that it no longer completely blocks the flow of mud, or in the case of blowout prevention: reservoir fluids.
Remote actuators currently use pneumatics and/or hydraulics to create the motive force that applies torque to the actuator/DSSV interface. In most cases, a linear motion is translated to a rotational motion through the use of racks and pinions or linkages.
In order to be able to deliver the torque to the DSSV stem, the actuator must be attached to the DSSV thus rotating when the DSSV is rotating. Therefore, delivery of pneumatic or hydraulic pressure to the actuator becomes problematic. The current methods of overcoming the delivery of pressure from a stationary source to a rotating actuator is through a hydraulic/pneumatic union or isolation of the actuators force generating mechanisms: typically hydraulic/pneumatic cylinders.
The advantage of using hydraulic unions is that they are very compact, very efficient, and very powerful. Full hydraulic pressure can be redirected through these devices and delivered directly to the hex drive shaft either through racks and pinions or through linkages. In this mode of design, all the actuator's force generating components can be internalized within the actuator body. The internalizing of the force generating components (typically racks and pinions) allows the actuator to remain relatively small, in comparison to other styles of actuators, while still delivering comparable torque. As well, as all the force components are internalized, the possibility of damage is greatly reduced improving reliability. In addition the union can be designed to operate as a plain bearing for the rotational component, eliminating the need for costly bearings and again saving space.
However, one draw back of the hydraulic union method is the design and use of small cross section hydrodynamic seals that seal oil glands between the stationary part of the actuator and the rotating part. The hydrodynamic seals provide positive sealing, due to seal compression, while the actuator remains stationary, but allow small amounts of oil to bypass when creating a dynamic seal. The bypassing oil ensures that the seal face remains lubricated, effectively creating a short journal bearing. The lubrication significantly reduces friction between the seal and the rotating member thereby extending seal life. Over time, this seepage and the combined inevitable seal wear from operation will escape to the environment, as collection and reuse methods are typically not incorporated into the actuator design.
The hydraulic fluid between the seal and the rotating member is subjected to high shear rates which in turn generate heat that is difficult to dissipate due to the actuators high thermal mass and small surface area. Further, if the hydraulic pressure to function the actuator acts on the seals while the actuator is rotating, the seals increase their facial surface force and act as a brake on the rotating member. Thus, heat generation and seal wear increase significantly.
In order to overcome leakage from the dynamic seals and the associated heat generation, some actuators have isolated the force generation by moving the hydraulic or pneumatic cylinders to the exterior non-rotating portion of the actuator. The external cylinders deliver a force to a moveable sleeve, isolated by bearings systems, which in turn drive linkages to create the torque at the actuator/DSSV stem interface.
The isolation of the cylinders often results in a larger less rigid actuator than the hydraulic union type due to the mounting methods of the cylinders and internal clearances required between the axially shifting sleeve(s). The reduction in rigidity results in accelerated wear of the joints that connect the cylinders to the non-moving part. As well, any linkages that are used to supply torque to the interface between the actuator/DSSV often develop significant unintended clearances. The increased wear at joints of the linkages and cylinders leads to inaccurate functioning of the DSSV, i.e. the DSSV is not moved from full open to full close when the actuator is moved through its range of motion.
Linkages are typically not as efficient as rack and pinion designs, and do not possess the same amount of mechanical advantage. In addition, because of their low mechanical advantage, linkages can be susceptible to moving without being actuated, as the vibration associated with drilling has been known to cause these linkages to move under their own weight and inadvertently close the valve during drilling cycles.
Regardless of the actuator style, the output torque is often limited by the size of hydraulic or pneumatic cylinders that can be incorporated into the design and their respective radial offset location from the axis of the DSSV's crank center. In the case of the externally mounted cylinders, the cylinders usually have a small diameter with a thin wall in order to keep the overall actuator size to a minimum. The small thin walled cylinders have limited pressure retention, thus the output force is also limited. The union style actuators typically do not suffer from the same pressure limits to their force generation components. However, as the force generating components are internal to the small diameter bodies, the offset distance between the force generation and the crank center of rotation is severely limited.
For any DSSV, the correct alignment of the ball in the open and closed position is critical to optimal valve life. Without correct alignment in the open position, the leading edge of the ball and the trailing edge of the lower seat will be exposed to abrasive mud flow, causing premature wear and potentially vortices that can accelerate erosion. The resulting deflected flow path and resulting accelerated erosion can lead to premature failure.
As the alignment of the ball is critical for valve service life, most remote actuators rely on the valve's internal stops to set the alignment of the ball. Without the internal stops, most actuators would provide excess rotational motion thereby allowing the ball to over travel in both the open and close positions.
Since the DSSV stem internal stops are used, the stops often get damaged (resulting in misalignment of the ball) from the high contact stresses that the actuator's output torque generates. Very few actuators have a provision for adjusting the actuators output motion limits. This adjustment would allow the actuator to correct the balls alignment within the valve without the need to perform costly repairs on the valve itself.
It is, therefore, desirable to provide an for a DSSV that overcomes the shortcomings of the prior art by eliminating the need for a hydraulic union thus eliminating the leakage and seal wear problems that are associated with prior art designs.